Anthropogenic emissions drive elevated HCl and particulate chloride in polluted urban air

Particulate chloride (Cl ⁻ ) and hydrogen chloride (HCl) strongly influence aerosol hygroscopicity, acidity, and visibility in polluted urban atmospheres, yet they remain poorly represented in chemical transport models. We quantify the role of anthropogenic emissions in driving elevated Cl ⁻ and HCl over Delhi by implementing anthropogenic HCl and Cl ⁻ emissions in WRF-Chem and simulating post-monsoon, winter, summer seasons. We show that the long-standing underestimation of particulate chloride in previous modeling studies over Asian urban environments arises from inadequate representation of gas-particle partitioning under high aerosol number concentrations. Revising the thermodynamic equilibrium constant using observation-constrained partitioning improves model performance, enabling accurate simulation of the observed magnitude and seasonal variability of particulate chloride. Observed HCl concentrations over northern Indian cities are among the highest reported globally for urban environments. Local anthropogenic emissions of Delhi dominate chloride formation, with open waste burning contributing 34% and 44% during the post-monsoon and winter seasons, respectively. In contrast, regional agricultural residue burning contributes only ~ 14% within Delhi, despite enhancing chloride levels in nearby urban centers. These results establish anthropogenic chlorine emissions as a dominant driver of urban aerosol chemistry and underscore the need to revise chloride representation in chemical transport models for polluted inland cities. Main Text